A GATE ARRAY or UNCOMMITTED LOGIC ARRAY (ULA) is an approach to the
design and manufacture of application-specific integrated circuits
(ASICs), using a prefabricated chip with active devices like
NAND-gates, that are later interconnected according to a custom order
by adding metal layers in the factory.

DESIGN

A gate array circuit is a prefabricated silicon chip circuit with no
particular function, in which transistors , standard NAND or NOR logic
gates , and other active devices are placed at regular predefined
positions and manufactured on a wafer , usually called a _master
slice_. Creation of a circuit with a specified function is
accomplished by adding a final surface layer or layers of metal
interconnects to the chips on the master slice late in the
manufacturing process, joining these elements to allow the function of
the chip to be customized as desired. This layer is analogous to the
copper layer(s) of a printed circuit board (PCB).

Gate arrayGate array master slices are usually prefabricated and stockpiled in
large quantities regardless of customer orders. The design and
fabrication according to the individual customer specifications may be
finished in a shorter time compared with standard cell or full custom
design. The gate array approach reduces the mask costs, since fewer
custom masks need to be produced. In addition, manufacturing test
tooling lead time and costs are reduced, since the same test fixtures
may be used for all gate array products manufactured on the same die
size. Gate arrays were the predecessor of the more advanced structured
ASIC ; unlike gate arrays, structured ASICs tend to include predefined
or configurable memories and/or analog blocks.

An application circuit must be built on a gate array that has enough
gates, wiring and I/O pins. Since requirements vary, gate arrays
usually come in families, with larger members having more of all
resources, but correspondingly more expensive. While the designer can
fairly easily count how many gates and I/Os pins are needed, the
amount of routing tracks needed may vary considerably even among
designs with the same amount of logic. (For example, a crossbar switch
requires much more routing than a systolic array with the same gate
count.) Since unused routing tracks increase the cost (and decrease
the performance) of the part without providing any benefit, gate array
manufacturers try to provide just enough tracks so that most designs
that will fit in terms of gates and I/O pins can be routed. This is
determined by estimates such as those derived from Rent\'s rule or by
experiments with existing designs.

The main drawbacks of gate arrays are their somewhat lower density
and performance compared with other approaches to ASIC design. However
this style is often a viable approach for low production volumes.

HISTORY

Sinclair Research ported an enhanced ZX80 design to a ULA chip for
the ZX81 , and later used a ULA in the
ZX Spectrum . A compatible chip
was made in Russia as T34VG1.
Acorn Computers used several ULA chips
in the
BBC Micro , and later a single ULA for the
Acorn Electron .
Many other manufacturers from the time of the home computer boom
period used ULAs in their machines.
Ferranti in the UK pioneered ULA
technology, then later abandoned this lead in semi-custom chips. The
IBM PC took over much of the personal computer market, and the sales
volumes made full-custom chips more economical. Commodore's Amiga
series used gate arrays for the Gary and Gayle custom-chips, as their
code-names may suggest.

Designers still wished for a way to create their own complex chips
without the expense of full-custom design, and eventually this wish
was granted with the arrival of the field-programmable gate array
(FPGA), complex programmable logic device (CPLD), metal configurable
standard cells (MCSC), and structured ASIC. Whereas a ULA required a
semiconductor wafer foundry to deposit and etch the interconnections,
the FPGA and CPLD had programmable interconnections. Today's approach
is to make the prototypes by FPGAs, as the risk is low and the
functionality can be verified quickly, but for production FPGAs are
very expensive, power hungry, and in many cases do not reach the
required speed. To address these issues, several ASIC companies like
BaySand, Faraday, Gigoptics and others offer FPGA to ASIC conversion
services.

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